IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Tribology Tuesday Sessions
       Session TR+MM-TuP

Paper TR+MM-TuP7
Single-Asperity Nanotribology and Nanorheology of Thin Poly(dimethylsiloxane) Films

Tuesday, October 30, 2001, 5:30 pm, Room 134/135

Session: Poster Session
Presenter: S. Tan, University of Minnesota
Authors: S. Tan, University of Minnesota
G. Haugstad, University of Minnesota
W.L. Gladfelter, University of Minnesota
Correspondent: Click to Email
The lubricating and non-stick characteristics of poly(dimethylsiloxane) (PDMS) have been exploited for years. As with many tribological systems, technological application led scientific understanding: experimental methods were not available to precisely control intersurface separation, contact geometry and loading conditions about individual surface asperities. In the last 15 years analytical tools have become available for this purpose. Uniquely, scanning force microscopy (SFM or AFM) employs a single microasperity and feedback-actuated tracking of surface topography. Thus "nanotribologists" now can investigate the fundamental unit of tribological systems: a single asperity with measurable radius of curvature, deforming into a material under measurable load, and sliding tangentially to the surface. Many SFM studies have involved simple systems, e.g. clean single-crystal inorganic surfaces or ordered ultrathin organic films lubricating such surfaces. Relatively little work has been on higher molecular weight, disordered films ranging in thickness from boundary lubrication (extremely thin) to bulk hydrodynamic (very thick). Yet real technological systems span this range. In the present work, SFM was employed to study nanotribology and nanorheology on poly(dimethylsiloxane) films varying from several to hundreds of nanometers thick. A wide range of molecular weights was examined, corresponding to bulk viscosities from 350 to 1,000,000 cS. Friction and pull-off forces were found to increase as a small fractional power of velocity over several decades. Similar, scaled-up behavior was observed using a cantilever-attached microsphere (R=10,000 nm) in place of the usual tip (R~10 nm). A dramatic increase in friction and adhesion was observed above a critical film thickness of approximately 3 Rg. Accompanying this increase was a qualitative signature of liquid-like behavior seen in force-versus-distance measurements.